The storage battery bank size theory is for a minimum effective safe domestic size with 2 adults 2.4 children, 2 cooking session of 1/2 an hour are available to no more than 15% DOD at constant current 8Kw(more a technical point). Note: remember all cooking should be done using float charge input method.
- meaning "8Kwh to 15% DOD" = minimum 4200ah total of "12v amp value" battery bank.
Batteries deteriorate capacity over time so if the 4200ah scenario is suitable in theory, in practice it should be around 5000ah minimum.
Incidentally, the previous would be an 8Kw(generators) hybrid power system although the same with downsizing can be a 6Kw(generators) hybrid(bu'! the wind turbine MUST be placed 15 meters up minimum not any lower!).

Battery Bank size for 12 years as 6000ah-12v-value is based on all "large jobs(washing machine, cookers, air-conditioner, heater, hot-water)" always having solar generation input during their operation with panel system of Pmax-3Kw and a 3Kw wind turbine too with all house large components downsized for max. 2Kw current draw (although if you do not use an air conditioner beyond 1Kw-electric input you do not need much downsize except the washing machine) .e.g. 1500w hot-water element , or 2000w portable grill oven, 2000w accessory hot plate, 2000w(electric-input) air conditioner[a portable air ice is best they are only 1000w and a giant 700L 2 door refrigerator is not an actual big item, they switch on for 5 minutes every 1/2 hour but worth 2Kwh-in 24hrs(2x however, scrapes large)] (no more than 3Kw constant current draw "normally/usually" at any one time. From 100% to 85% full (15% DOD) with no solar or wind input at 3Kw current draw takes 3 and a half hours - and that is a single days maximum discharge allowed making 6 hours of 3Kw constant current draw if solar and wind are present)! If any one other large item is switched on then one that is presently operating should be switched off!
NOTE: Below 8Kw generator capacity, the wind turbine should be placed a minimum of 15 meters high tower, and the rule is nothing in off grid is placed below 12 meter wind turbine tower (ref: Meteorology PBL and wind percentile measurement layers)!

For normal house appliances and no more than 3Kw(electric-input) air conditioner the generator needs to be 5K-solar and 3Kw rated wind turbine minimum (but no more than 5Kw current draw "normally/usually" at any one time - after dark, air-cond. should be on low or half).

(*)The above is because the 15% DOD maimum discharge theoretical marker is a safety point reserved for mid winter and mid summer.

NOTE: Of late in July 2015, "sealed GEL lead acid" long float life 12-15 years large number discharge (only to 10 - 20 percent DOD - down to 90-80 percent full)
have been cheaper to acquire or at least some companies can be fossicked with better trade deals for GEL lead acid high quality long life.

Because of this "sealed lead acid GEL type" are the better to bother acquire [price is usually $1.1 USD P/ampere ].

NOTE: Long float life is achieved by using the battery at a lower charge voltage and at or below 20 degrees Celsius.

"Depth Of Discharge (30% and 15%) Time Calculator for Kw of drawn Current(amps)" *Note: Quantifiers not available in C, Java or FORTRAN at this time - web script only!

As example next in this "DOD Time Calculator" for a battery bank.
If total battery bank storage amps at 12 volts is 80AH (almost 1Kwh total) (e.g. just one battery) the theoretical time to 30% DOD (1/3 discharge) is "17" minutes of 1Kw.
Then 3 x 17 = 51 minutes , 3 x 30% = 90% of the battery storage size.
The following javascript form calculates the quantity of time a battery bank has if it were being discharged at a constant Kw draw level of current(amperage) use.

The average all exclusively electrical energy supplied house in Australia is between 25Kwh to 35Kwh daily,
hence draw at "1Kw"(the select options field) down to 15% DOD should "hypothetically(not necessarily)" equal "at least" around "25 hours" in the decimal-fraction hour field(output answer) in the following form, relating the settings of AH(ampere hours) of battery bank filled into the "battery banks TOTAL 12v worth of AH's" and selection of "@ "X"Kw current draw" selector.

When batteries are added inline in series the volts are added together and raised, but the AH remain the same.
When batteries are put in parallel, tha AH's are raised but the volts remain the same.

For the storage quantity by 12 volt batteries the total storage is easiest by 12v.
If you have 80 of 12v batteries that are 80ah batteries that makes 6400ah.
So if all the 80 were parallel(never do that) the calculator will say you have at "1"Kw , "11.566265060240963" hours = 11.566265060240963 Kw/h's.
By the same point, if our 80ah 12 volt batteries were arranged like next, "8 lines of" --- "10 batteries in series" using 120v power system storage ,
and we used a calculation for 120v at current(ampere) draw of 1Kw it would be exactly the same!!! "11.566265060240963" hours = 11.566265060240963 Kw/h's. !!!

So you can build any voltage system you want "with 12 volt deep cycle accumulators" but voltage(by battery arrangement) does not and cannot change the quantity of electricity stored!
Just add all the 12v batteries AH ratings together for the total(equivalent to multiplying the AH rating by the number of batteries) , or if you have 2v single cell deep cycle accumulators(batteries), divide by six the total number of batteries and then multiply that number of accumulators by the AH rating they have(they should all be the same! e.g. 3000ah or 250ah or 80ah ... whatever e.t.c ...)

For an actual permanent off grid system, because "14000ah worth of 12v (25Kwh's-15%DOD)" size is too costly(for dedicated 10+ years), and as much the power system can use by management 1:1 ratio(half battery supply AND half wind/sun generator) of "battery bank" to "direct sun/wind generation" can be the day/night efficiency balance when added together, only 6000ah worth of 12v for normal grid mimic is required and around 4000ah for smaller systems the house has had its appliances remodeled for efficiency.

[ Amps and Watts calculator for System Voltages ]
Fill in one field only (numerals only) to calculate!
If you want to find the amps in various system voltages, simply change the volt selector after calculating!
IF you want to find watts "FROM AMPS" again, "set the voltage selector first before" "click-on-clear" a text entry field to rewrite in AMPS again and then using the Calculate button!!!
(Volts)
(Amps)
(Watts) [Watts is a measure of "job Energy" extremely "alike" Kilo-Joules or Kilo-Calories]

WARNING:

[ Always research GEL lead acid for discharges to price , apart normal fluid lead acid type , another is also PAM , it's usually MOQ is the deal problem but ask them ]
While the calculation above shows you how much storage in "the basic daily requirement must be available", it does not tell you much about the differences of "Lead Acid type batteries alone" for the purpose of lasting 12 years to 15% DOD , the following calculation can be fed the number of discharges from the specific models' data sheet to convert to years as a decimal fraction.

If the battery is a sufficient quality it will behave something alike double the 30%DOD discharges at 15%DOD.

However, generally only half of the total daily requirement is "one single discharge" for storage (ampere hours) purpose, and in heavy management tactic and modified household refit is calculated as only 1/3rd of the total storage requirement per single discharge for the 12 year period.

Calculate The number of Discharges(at whatever DOD - we're using 10% - 15% in this off grid model for 12 - 15 years) as days into years (approx 1 discharge is one day).

Number of Discharges to X% DOD discharges to Years

Calculate the "single battery AH size" and the "total quantity of batteries(accumulators)" required for the Battery Bank at System voltage (and "number of series lines" and "accumulators p/line")

[Warning! General rule of thumb: Never use a battery below 80ah unless the total generator is "below 6Kw" and or below 96 volt (note: 65ah -to- 80ah is just ok up to 8Kw and 240v) , never use an accumulator below 40ah unless the total generator is "below 3Kw"]

NOTE: The AH for each battery is the approximate size only, an actual product will have its manufacturers closest size. This is a rough guide only.

These are the common Lead Acid deep cycle types and efficiency varies immensely with ,
* price (above or below MOQ , company and set quantity) * Note, always buy "EXW" or "CIF"(particularly) only!!! (*sp-note: MOQ - Minimum Order Quantity - e.g. units: "sets" or "pieces")
* DOD depth of discharge ratings (shorter float life-span 15%DOD is similar to 30%DOD - long-life 15% is more double 30%)
* with temperature-float-lifespan (accumulator model life-span should be more than 10 years always at or above 20o Celsius!)
* local environment temperature affected "ah capacity".
* current draw "rate of discharge" affected "ah capacity".

In short, there can be any combination of mentioned above offered , but it is a matter of verifying the price , the model and "data sheet information" are effective at a 12 year job.

You need to be sure it will operate around 50% capacity at 10oC , the float life is more than 12 years at 20oC minimum , and discharge to 30% is at least 1600 discharges , and if a GEL.

In short always ask for an estimate of 15% DOD and 30% DOD and the data sheet, and it always requires a high quality "Lead Acid" to be economic enough.

*As explained later, there are ways of keeping the temperature of batteries at a level that will give more than 10 years float life and around 80-90% capacity to charge to.
RECAP NOTE: Remember as said before, only use a totally sealed battery type, NEVER ventilated to atmosphere!

Here is a "DATA SHEET Example" of a 200AH GEL lead acid battery" (insufficient version), their quality and specifications vary greatly alike the standard sealed fluid lead acid, however they generally have alike twice the discharges of a standard fluid type DOD to DOD comparison. This one is not sufficient because it rated below 10 years @ 20oCelsius

Here is a sufficient version 80ah-100ah GEL http://mdeng.en.alibaba.com which the "100 pieces M.O.Q." trade lot will be more expensive "per piece" if only 50 were the trade deal, but the piece quantity(50) is effectively perfect by discharges for the 120v or 240v(240v would require 60 pieces) 8Kw and 10Kw Hybrid power generator systems to between 30% and 15% over 12 years compared to the high quality lead acids full stop!